FORAtherm Photothermal Radiometry
FORAtherm – Non-destructive characterization of surfaces-near structures Photothermal radiometry is a contactless and nondestructive testing method for hardness measurements (e.g. hardness profiles of case-hardened steels). Other applications are measurements of coating thickness, coating adhesion or detection of grinding burns. Whenever surface layers are altered by mechanical, chemical or heat treatment, a characterization with photothermal radiometry should be considered. This technique is much faster than metallographic analyses making inline hardness inspection possible. Even complete hardness depth profiles can be reconstructed for laboratory purposes.
APPS/CONCEPT Typical applications
Principle of photothermal radiometry
Hardness testing • Measurement of case hardness depths (CHD) • Hardness depth profiling of coatings • Measurement of coating thicknesses • Detection of delaminated coatings • Detection of hidden corrosion
The photothermal measuring method is based on intensitymodulated or pulsed heating of surfaces by laser light. During excitation, the surface temperature is monitored by an infrared detector. The generated heat diffuses into the sample. The detector measures the resulting surface temperature, which depends on material parameters and sample geometry. A lockin amplifier extracts the phase information of the infrared signal. Using calibration measurements, the phase can be linked to certain material parameters like hardness or coating thickness. Therefore, the system needs to be calibrated with destructive measurements on sample material and geometry.
Other applications • Detection of grinding burns • Porosity quantification • Testing of welding joints
The measurement duration depends on the type of application: • Classification of case depths at a certain modulation frequency within 10 to 30 seconds • Reconstruction of a hardness depth profile within 15 to 40 minutes
FFT
IR detector
Modulated excitation
Laser
Case depth
Object
SPECIFICATIONS Laser Description Optical output power Wavelength Power supply Cooling
Fiber-coupled diode laser max. 25 W 810 nm 230 V, 16 A, 50 Hz Peltier
System Transimpedance amplifiers Digital two-phase lock-in amplifier PC for data acquisition and laser control Customer adapted software (FORAspect) Optional: scanning system
Detector Detector material Lens material Spectral response Cooling
Indium antimonide (InSb) Germanium (Ge) 2,0 - 5,5 µm Stirling cooler or liquid nitrogen
Measurement duration Fixed-frequency measurement (1 - 30 s) Frequency scan (10 s – 30 min) Measurement of case hardening depths (5 s – 1 min) Reconstruction of hardness depth profile (15 - 40 min) Good-bad-classification (1 – 10 s)
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